Deformation Characteristics of Rubber Waste Powder–Clay Mixtures

Abstract

With the increasing accumulation of rubber waste, the potential reuse of rubber fillers offers a promising solution to enhance the engineering properties of low-plasticity soils while promoting environmental sustainability. In this study, the effect of rubber waste powders (RWPs) on the consolidation and deformation properties of low-plasticity clay soil (CS) was investigated using a fully automated consolidation testing procedure for clay–rubber mixtures. The study involved adding 2% up to 30% RWPs to Tehran clay, and various parameters were evaluated through consolidation, compaction, and uniaxial strength tests. The results revealed that the consolidation volume of the mixture differed from that of the CS due to the elastic nature of the rubber wastes (RWs). To achieve higher precision, a new equation was proposed to determine the void ratio, along with modified e-log p’ curves for the clay–rubber mixture. Furthermore, the addition of RWPs to the CS resulted in moderated free swelling of the soil while enhancing ductility, compression index (Cc), swelling index (Cs), and recompression index (Cr). However, it was observed that the strength and modulus of elasticity of the mixture decreased with the increase in rubber content. Considering the variations in geotechnical parameters with different rubber contents, the appropriate rubber content can be selected based on specific applications in soil and rubber mixtures, considering the required geotechnical parameters. This study highlights the potential applications of RWPs as a material in civil and geotechnical engineering projects, providing valuable insights for sustainable and eco-friendly engineering practices.

1. Introduction

Human societies are currently grappling with the challenges posed by rubber waste (RWs), which, despite being recyclable, often ends up in landfills, causing environmental concerns. To address this issue, various studies have explored the use of RWs to enhance the engineering properties of different materials [1,2,3,4,5,6,7]. Previous research has highlighted the favorable attributes of RWs, such as high frictional strength, durability, flexibility, and compressive/tensile strengths, making them suitable for geotechnical engineering applications [5,8,9]. The unique elasticity and damping properties of rubber make it a promising candidate for improving the seismic performance of clay soil (CS) [10,11]. Fibers are effective in geotechnical properties of clay soils [12]. Similar to fibers, which have proven effective in enhancing the properties of fine-grained soils, rubber is expected to positively impact the geotechnical properties of soils, much like fiber-reinforced materials ones [13,14,15,16]. Incorporating different materials for soil improvement has been an area of significant interest, with several methods and studies exploring this avenue [17,18]. While research on the effect of rubber on geotechnical properties has been explored for sand–crumb rubber mixtures [19,20,21,22,23], there is limited information on its impact on some characteristics of clay soils. Some studies have indicated that the uniaxial strength of clay–rubber mixtures may decrease with rubber contents exceeding 4% [24]. Nevertheless, the use of homogenous rubber powder in the soil has shown positive effects on the repeatable properties of the clay–rubber mixture. Microscopic examination reveals that the rubber grains are relatively longitudinal and contribute to reinforcing the clay–rubber mixture, improving its flexibility and ductility. The mixture exhibits a transition from brittle to ductile failure behavior [25]. Evaluating clay–rubber mixtures through various tests, including direct shear, uniaxial, and triaxial tests, has demonstrated the promising geotechnical properties of such mixtures and their suitability for civil engineering projects [10]. Other studies have also examined the use of triaxial tests to assess the potential of CS–RWs mixtures for geotechnical engineering applications [9,11].

Clay soils are of great importance in geotechnical engineering and extensive research has been conducted to investigate their geotechnical properties [26,27,28,29,30]. Few studies have thus far been completed on the consolidation and the swelling properties of clay–rubber mixtures. For instance, a study had been conducted on the impact of RWs on the swelling properties of two different expansive clay soils in Algeria. The swelling properties of the specimen had been further assessed via the consolidation test under loading and unloading. According to the study results, the swelling properties, the swelling pressure, and the time to reach the maximum swelling in highly expansive clay had declined following an increase in the rubber content. The results were also suggestive of the applicability of this mixture in civil projects [31]. The consolidation test on expansive clay–rubber mixtures had comparably indicated that adding 15% rubber could cause a reduction in the swelling properties of clay. In contrast, the mixtures had shown compaction equal to that of the pure soil, and additionally, the strength had been slightly boosted by adding rubber to the soil [13]. Consolidation testing of a high-plasticity clay–crumb rubber mixture had also established that the compaction of the mixture had decreased by adding 5–10% rubber to the soil, but it had started to increase at higher contents [32]. The addition of rubber to clay samples reduces the post-peak loss of strength, making the soil mixture more flexible, elastic, and less brittle. Different shapes of rubber in the mixture contribute to controlling cracking. Mixtures containing granular rubber show higher resistance and achieve the highest modulus of elasticity compared to other rubber shapes at the same percentage of rubber content. The type of failure in the soil and rubber mixture varies based on the shape and amount of rubber used, including shear plane failure, shear wedge failure, shear area failure, multiple vertical cracks, multiple diagonal cracks, and bulging failure [16]. The damping ratio of the clay–rubber mixture increases with a decrease in rubber grain size. Up to about 10% rubber content, the damping ratio increases, but with a further increase in rubber content, the damping ratio decreases. The shear modulus of the soil–rubber mixture increases with an increase in grain size, but it decreases with an increase in the amount of rubber content. Reducing the shear strain amplitude in cyclic tests increases the shear modulus and decreases the damping ratio of the mixture [9]. Increasing rubber content in clay increases the elastic, plastic, and cumulative plastic strains, particularly when using smaller rubber grains [25].

Table 1. Some recent studies on clay–rubber mixtures.

Reseach Topic	References	RWs Content (%)	RWs	Soil	RWs Size (mm)
Mechanical properties	[32]	5, 10 & 15	Chips *	CH	4.75–2
Construction material	[5]	2.5, 5, 7.5 and 10%	Granulate and fiber	CL	0.8–2 mm, fiber
Geotechnical properties	[10]	0, 2, 4, 6, 8, 10, 20 & 30	4 forms of rubber	CL	Different size
Dynamic properties	[9]	<1 mm and 1–5 mm	Granular	Tehran Clay	0, 2, 4, 6, 8, 10, 20 and 30%
Strength Characteristics	[11]	5, 10 and 25%	Granular	Red clay & Kaolin	0.1–1 mm and 1–5 mm
Failure analysis	[16]	10, 20 and 30%	4 forms of rubber	CL	Different size
Elaspo-plastic charactristics	[25]	Up to 30%	Granular	Tehran clay	<1 and 1–5

* Sand up to 90% is used as an additive; CH: High Plasticity Clay; CL: Low Plasticity Clay.

summarizes some recent studies on clay–rubber mixtures.

A review of the literature here explains that although a major portion of clay in the nature and civil projects are of low-plasticity type where rubber waste can be used as filler materials, no comprehensive study has been so far fulfilled on the deformation behavior of clay–rubber mixtures. It has been mentioned in various studies that the clay soil in the south of Tehran, Iran is known to be mainly composed of low-plasticity clay [33,34]. In this study, consolidation tests were accordingly performed on Tehran clay to characterize the properties of the mixture by adding rubber powder. Considering the need for understanding the strength properties of the mixture concerned, the uniaxial test was practiced, and the optimum moisture content (OMC) and the maximum dry density (MDD) of the mixture were experimentally determined. Accordingly, the aim is to reuse RWs as filler materials in civil and geotechnical engineering and reduce environmental problems caused by such wastes.

2. Materials and Methods

2.1. Materials

2.1.1. Tehran Clay (CS)

Tehran is situated at the base of the Alborz Mountains, characterized by young alluvial deposits formed through extensive erosion from the Alborz Mountains along various fault lines. The city can be divided into two main regions: the pediment in the north and the northern plains of central Iran in the south. The alluvial deposits of southern Alborz extend from north to south and are typically found over impermeable beds, covered with diverse alluvial sediments. Sedimentological studies of the alluvial deposits in and around Tehran indicate that they are formed by seasonal river activity and floods originating from the southern slopes of the Alborz Mountains. As described above, clay lenses are present throughout most parts of Tehran. Additionally, the soil in the northern regions of Tehran tends to be more coarse-grained while in the southern parts, it is characterized by fine-grained alluvium. For this study, experiments have been conducted on soil samples collected from the southern parts of Tehran.

Local investigations and site studies have always been of interest to researchers [16]. For this study, clay soil (CS) samples were collected from the southern region of Tehran, which were predominantly of the low-plasticity type [33,34,35]. The choice of this natural CS was made with the intention of practical applicability for the results obtained. The samples were taken using a core cutter from a depth of 2 m, ensuring they were free from clay loam and made ground. The sampling location and waste tires near the trash bin in Tehran is also shown in Figure 1. XRD and XRF tests were carried out in the mining faculty of Amirkabir University of Technology and according to conventional methods. X-ray diffraction (XRD) analysis of the Tehran clay (Figure 2) revealed that the dominant clay mineral in this soil is kaolinite and, along with its chemical properties and constituent elements obtained from X-ray fluorescence (XRF), revealed that the clay primarily contains calcium oxide (CaO) and ferric oxide (Fe₂O₃) (Table 2). Additionally, the grading curve of the soil was determined using the Standard Test Method for Particle-Size Analysis (ASTM D422) [36] (Figure 3). The results showed that 65% of the soil passed through sieve #200.

The physical properties of the CS are presented in Table 2. According to the Unified Soil Classification System (USCS) (ASTM D2487) [37], the CS falls under the category of low-plasticity soil (CL).

Table 2. Engineering geological and geotechnical properties of Tehran clay and rubber waste.

Geotechnical Properties of CS
Properties	ASTM Standard	Values
Specific gravity	D 854 [38]	2.65
Liquid limit (%)	D 4318 [39]	34
Plastic limit (%)	D 4318 [39]	14
Plasticity index (%)	D 4318 [39]	20
Soil type (USCS)	D 2487 [37]	CL
Maximum dry unit weight (kN/m3)	D698 [40]	16.3
Optimum moisture content (%)	D698 [40]	18.5
Fine percent (%)	D422 [36]	65%
Chemical Composition of CS
Chemical Components	Percentage (%)	Test Type
SiO2	55	XRF
Al2O3	10.5
Fe2O3	8
CaO	18.6
MgO	5.4
L.O.I (loss of ignition)	2.5
Chemical Composition of RWPs
Components	Percentage (%)	Test Type
Carbone	86.8	XRF
Oxygen	9.3
Zink	1.95
Sulfur	1.4
Magnesium	0.23
Aluminum	0.12
Silicon	0.2

Table 2. Engineering geological and geotechnical properties of Tehran clay and rubber waste.

Geotechnical Properties of CS
Properties	ASTM Standard	Values
Specific gravity	D 854 [38]	2.65
Liquid limit (%)	D 4318 [39]	34
Plastic limit (%)	D 4318 [39]	14
Plasticity index (%)	D 4318 [39]	20
Soil type (USCS)	D 2487 [37]	CL
Maximum dry unit weight (kN/m3)	D698 [40]	16.3
Optimum moisture content (%)	D698 [40]	18.5
Fine percent (%)	D422 [36]	65%
Chemical Composition of CS
Chemical Components	Percentage (%)	Test Type
SiO2	55	XRF
Al2O3	10.5
Fe2O3	8
CaO	18.6
MgO	5.4
L.O.I (loss of ignition)	2.5
Chemical Composition of RWPs
Components	Percentage (%)	Test Type
Carbone	86.8	XRF
Oxygen	9.3
Zink	1.95
Sulfur	1.4
Magnesium	0.23
Aluminum	0.12
Silicon	0.2

2.1.2. RWPs

Rubber waste powders (RWPs) obtained from crushing tire waste were used to reinforce the CS. The grading diagram of the RWPs is shown in Figure 3. According to the USCS for grain size (ASTM D2487) [37], the rubber powder is classified as poorly graded sand (SP), with curvature and uniformity coefficients of 2.7 and 3, respectively. The rubber grain size ranges from 0.1 to 1 mm, approximately similar to that of sandy clay. Table 2 provides the chemical properties of the rubber powder, which mainly consists of carbon and hydrogen. Additionally, Figure 4a displays waste tires and RWPs, and Figure 4b also displays a high-resolution image (40×) of the RWPs under an optical microscope. The elongated shape of the rubber particles, with an approximate length-to-width ratio (aspect ratio) of 1:1, contributed to the reinforcement of the CS by the rubber powder.

2.1.3. CS–RWP Mixture Preparation

As test results can be influenced by specimen preparation and rubber content, various rubber contents have been used in the literature [9]. At high rubber contents, the behavior of the mixture is primarily governed by the rubber, resulting in a significant decrease in the geotechnical properties of the mixture [16]. Consequently, this study employed a range of rubber contents from 0 to 30 wt% (relative to the dry soil) to investigate the impact on the specimens. Considering that low percentages of rubber are more useful in engineering works [10,25], in RWP contents less than 10%, the steps of increasing RWPs are 2 by 2 (2, 4, 6, 8 and 10%) and due to the lower use of higher amounts of RWPs, in rubber contents more than 10%, in order to investigate the engineering behavior of this mixture, 20% and 30% RWPs have also been used in this study. Various tests were conducted to characterize the specimens.

Table 3. Mixture designation and proportions.

Sample Number	Sample Type	Sample Code	Test
1	Pure clay soil	C100P0	U.C.S, Com., Con.
2	Clay soil + 2% rubber powder	C98P2	U.C.S, Com., Con.
3	Clay soil + 4% rubber powder	C96P4	U.C.S, Com., Con.
4	Clay soil + 6% rubber powder	C94P6	U.C.S, Com., Con.
5	Clay soil + 8% rubber powder	C92P8	U.C.S, Com., Con.
6	Clay soil + 10% rubber powder	C90P10	U.C.S, Com., Con.
7	Clay soil + 20% rubber powder	C80P20	U.C.S, Com., Con.
8	Clay soil + 30% rubber powder	C70P30	U.C.S, Com., Con.
9	100% rubber	C0R100	Con.
Test Types:
U.C.S.:	Unconfined compressive strength (ASTM D2166) [41]	Com.:	Standard compaction test (ASTM D698) [40]
Con.:	Consolidation test (ASTM D2435) [42]

provides details on the specimen codes, including soil and rubber contents, along with the tests performed on each specimen. Each specimen in Table 3 is identified by two letters and a number. The first letter and number represent the CS type and content, respectively, while the second letter and number indicate the rubber content in the mixture. For instance, C90P10 represents a mixture containing 90% CS and 10% RWPs. To prepare the specimens, the CS and RWPs were thoroughly mixed. After adding the optimum moisture, the mixture was placed in plastic bags for 24 h to ensure a uniform distribution of moisture. Subsequently, the specimens were constructed in the consolidation mold considering the maximum dry density (MDD) of the soil.

Observing the clay–rubber mixtures under a binocular allowed for capturing high-resolution images of these voluminous objects. Figure 5 exhibits views of the soil–rubber-water mixtures taken using a binocular, optical microscope, and SEM microscope. As depicted, there is cohesion between the CS and rubber particles. It is worth noting that clay minerals, including kaolinite, also contribute to water absorption, fostering good cohesion between CS, water, and rubber, leading to the favorable geotechnical performance of the mixture.

2.2. Laboratory Study

In this study, the deformation characteristics of the soil–rubber mixture were thoroughly investigated through consolidation and compaction tests. Additionally, unconfined compressive strength (UCS) tests were employed for further investigation. Below is a description of how these tests were performed.

2.2.1. Consolidation Test

Consolidation and swelling (rebound) tests were conducted on the specimens using a fully automated testing machine in accordance with ASTM D2435 [42] standards. The use of an automated machine allowed for precise and accurate recordings during each stage, eliminating manual errors and enhancing the reliability of the results. Figure 6a depicts the testing machine and the consolidation test samples used in the experiments. Figure 6b presents test samples containing various rubber contents (from 0 to 30%) following the consolidation test. As illustrated, the mixture color turns darker once the rubber content is augmented. The experiments were accordingly carried out at swelling and consolidation stages. During the swelling stage, the specimen was subjected to a pressure of 5 kPa to allow for free swelling, and the swelling rate was recorded at different time intervals. The swelling percentage at various time points and the final swelling percentage after reaching equilibrium were also calculated. For the consolidation stage, the specimen was subjected to incremental pressures of 12.5, 25, 50, 100, 200, 400, 800, and 1600 kPa, each for 24 h, with automatic recordings at specific intervals as per the standard procedure. To assess the unloading behavior, the unloading steps were conducted from 1600 kPa to 800, 400, 200, and 100 kPa, respectively, with displacement recordings taken up to 4 h before the next unloading step.

Considering the presence of rubber in the mixture, the compaction behavior of the specimens differed from that of the pure soil. To evaluate the effect of rubber on the consolidation and swelling, three consolidation tests were conducted on different specimens. In the first experiment, the pure soil was tested following saturation (Figure 7a,b). The second test focused on the consolidation and swelling of the pure rubber, given its high elasticity (Figure 7c,d). The third experiment involved testing clay–rubber mixtures with varying clay contents, as specified in Table 3. The rubber waste powders (RWPs) with different levels (contents) were mixed accordingly. Figure 7 illustrates the initial and final void ratios obtained from these tests, providing valuable insights into the behavior of the clay–rubber mixtures.

2.2.2. Unconfined Compressive Strength (UCS)

To assess the strength and settlement characteristics of the specimens and facilitate the design of suitable mixtures, the uniaxial test was conducted. This test aimed to evaluate the uniaxial strength of the specimens without applying a confined load, following the ASTM D2166 [41] standard for cohesive soils. The specimens were loaded using conventional methods at five different layers of equal thickness, with a strain-loading rate of 1 mm/min [13].

2.2.3. Compaction Test

The compaction test was conducted to determine the Optimum Moisture Content (OMC) and the Maximum Dry Density (MDD) of the soil. This test was essential for practical applications in construction projects. The specimens were prepared under optimal conditions and then subjected to testing. As shown in Table 3, the standard compaction test was performed on various specimens following the ASTM D698 [40] standard (12,400 ft-lbf/ft³ [600 kN-m/m³]).

3. Results and Analysis

In addition to discussing the effect of rubber on the Optimum Moisture Content (OMC) and Maximum Dry Density (MDD), the impact of rubber on the consolidation and swelling properties of the CS was also examined. Furthermore, the strength properties of the specimens were evaluated, and their uniaxial strength is analyzed in this section.

3.1. RWPs Effect on MDD and OMC

Figure 8 shows the standard compaction diagrams of the specimens containing different levels of RWPs. It is evident that the Maximum Dry Density (MDD) of the pure soil is 16.3 kN/m³, and the Optimum Moisture Content (OMC) is 18.5%. As the rubber content is increased, the OMC also increases while the MDD decreases. For instance, at a rubber content of 30%, the MDD and OMC are 13.2 kN/m³ and 21%, respectively. This reduction in mixture density can be attributed to the lower density of the rubber powder compared to the pure CS. Adding rubber to clay reduces the unit weight of the soil, which can significantly reduce the pressure caused by embankment in the mixture and lower implementation costs. Similar trends have been reported in the literature for the MDD and OMC in clay–rubber mixtures [3,10,13,35].

3.2. RWP Effect on Stiffness

3.2.1. Swelling Potential

After conducting the consolidation tests, the swelling strains were evaluated, and the results are presented in Figure 9. As the rubber content is increased, the mixture color becomes darker. Figure 9 displays the swelling strains of the specimens with different rubber levels. It is evident that the specimens containing rubber powder exhibit lower swelling strains compared to the natural CS, indicating a reduction in the swelling strain of the mixture. Figure 10 illustrates the changes in the free swelling as a function of the rubber content. The swelling strain of the pure soil after 24 h is 45% while the soil containing 2%, 4%, 6%, 8%, 10%, 20%, and 30% rubber exhibits swelling strains of 40%, 42%, 39%, 45%, 39%, 31%, and 31%, respectively. It is observed that the use of rubber causes a reduction in the free swelling of the natural CS and ultimately improves the geotechnical properties of the mixture induced by clay swelling. This reduced swelling in the rubber-containing mixtures can be attributed to the rubber’s lack of water absorption and, consequently, no swelling. Moreover, the clay content in the mixture decreases with an increase in the rubber content, contributing to the overall drop in clay swelling.

3.2.2. E-Log P’ Curves

The e-log p’ curves were employed to assess the effect of rubber on the consolidation properties of the pure soil. Figure 11 presents the e-log p’ curves for the clay–rubber mixture at different rubber contents, plotted according to ASTM D2435 [42] standards, assuming the same void ratio for both the clay–rubber mixture and the natural CS. A downward trend is further observed with increasing the rubber content, and the void ratio of the specimens decrease relative to the natural clay.

An interesting question arises: “Do the void ratios of the natural clay and the clay–rubber mixture follow the same relations at the beginning and at the end of each consolidation stage?” Initially, the void ratios of both pure clay and clay–rubber mixtures (^e_SO and ^e_TO) follow the same relation and can be calculated from Equations (1) and (4) in

Table 4. Equations for void ratio calculation.

Equations for calculation of void ratio for pure soil	No.
$e_{S0}=\frac{V_V}{V_S}=\frac{H_v·A}{H_S·A}=\frac{H-H_S}{H_S}=\frac{H}{H_S}-1=\frac{H·A·G_{S·}{\gamma }_w}{W_S}-1$	(1)
$e_{Si}=\frac{V_{VSi}}{V_{SSi}}$	(2)
$e_{Si}=e_{S(i-1)}-{∆e}_{Si}$	(3)
Equations for calculation of void ratio for soil and rubber mix	No.
$e_{T0}=\frac{V_{VT}}{V_{ST}}=\frac{H_{vT}·A}{H_{ST}·A}=\frac{H_T-H_{ST}}{H_{ST}}=\frac{H_T}{H_{ST}}-1=\frac{H·A·G_{ST·}{\gamma }_w}{W_T}-1$	(4)
$e_{Li}=\frac{V_{VL}}{V_{SL}}$	(5)
$e_{Ti}=e_{si}-{N_i·e}_{Li}$ (New equation for soil and rubber mixture)	(6)

based on the G_S (specific gravity). However, there are differences in calculating the void ratios after loading stages. During the consolidation of the natural soil, the applied pressure primarily removes water from the cavities, and the elastic volume reduction of the soil grains does not significantly contribute to the overall volume reduction. On the other hand, in the case of rubber-containing specimens, part of the applied pressure results in the elastic compaction of the rubber particles. Therefore, the observed volume reduction includes both the lowered volume of the pores due to elastic volume reduction of the rubber and the reduction from water removal. This distinction needs to be considered in calculations, as conventional methods may lead to void ratios that are higher than the actual ones and should be corrected.

To address this issue, a new correlation was proposed by performing the same test on a pure rubber specimen. Figure 12 illustrates the e-log p’ curve for the pure rubber. As the pure rubber lacks pores, the volume reduction during consolidation loading is attributed to the reduction in the elastic volume of the rubber. These results were utilized to correct the void ratios of the mixtures. The void ratio was corrected at each applied stress based on the rubber content and the elastic volume reduction of the rubber, employing Equation (6) in Table 4 to calculate the void ratio of the clay–rubber mixture.

Figure 13a displays the new e-log p’ curves obtained from the proposed correlation. To compare the curves obtained with the new correlation and those calculated by conventional methods, both modified and unmodified curves are shown. As depicted, the modified curves lie above the unmodified curves, and the gap between them widens with increasing rubber content. It should be noted that if the curves are not modified, the void ratio might become negative at high rubber contents. By using the new correlation, the void ratio remains positive, ensuring a more accurate representation of the behavior of the clay–rubber mixture.

3.2.3. Compression, Swelling, and Recompression Indices

Various parameters were obtained from the consolidation test, and three of them, namely the compression index (Cc), the swelling index (Cs), and the recompression index (Cr), are particularly important. The effect of adding rubber to these parameters was therefore investigated, and the results are presented in Figure 13. Figure 13a displays the new e-log p’ curves. Figure 13b illustrates the effect of rubber powder on the Cc. The Cc of the CS increases slightly as the rubber content is enhanced. The rise in Cc is limited, with values changing from 0.224 for the pure soil to 0.227, 0.226, and 0.23 for CSs containing 10, 20, and 30% rubber powder, respectively. This increase indicates a slightly larger settlement of the mixture compared to the pure soil. However, the impact on the CS settlement is not significant. Interestingly, the mixture containing 2% rubber powder shows the lowest Cc among the other mixtures. The improvement in Cs with increasing rubber content can be attributed to the higher flexibility and elasticity introduced by the rubber powder, resulting in better settlement behavior of the specimen.

Figure 13c shows the effect of rubber powder on the Cr. The rubber powder significantly affects this parameter, especially at the beginning of the e-log p’ curve. The impact of the rubber powder on Cr is more pronounced than on Cc. The Cr of the clay–rubber mixture increases from 0.009 for the pure soil to 0.017, 0.039, and 0.057 for mixtures containing 10, 20, and 30% rubber powder, respectively. The lowest Cr value of 0.01 was observed for the mixture containing 2% rubber powder. The increase in Cr in the clay–rubber mixture compared to the pure soil is attributed to the elasticity of the rubber powder, which significantly influences the consolidation settlement of the soil.

Figure 13d displays the effect of rubber powder on Cs. The Cs increases with a higher rubber content, indicating greater swelling during the unloading stage. The swelling index increases from 0.029 for the pure soil to 0.048, 0.07, and 0.082 for mixtures with 10, 20, and 30% rubber powder, respectively. The lowest Cs value of 0.043 was observed for the mixture containing 2% rubber powder. Similar to Cc and Cr, the upward trend in Cs relative to the pure soil can be attributed to the elastic properties of the rubber powder.

3.3. RWP Effect on UCS

Understanding the strength and the settlement conditions is of utmost importance in geotechnical designs. To investigate the strength of the clay–rubber mixtures in this study, the strength of the specimens was measured through uniaxial tests. Figure 14a shows the strength of the clay–rubber mixtures containing different rubber levels. As depicted, the stress decreases while failure strains elevate in the stress–strain diagram with a growth in the rubber content. In other words, as the rubber content is augmented, the failure behavior changes from brittle to ductile, leading to improved ductility of the mixture.

Figure 14b displays the changes in the uniaxial strength once the rubber content is added. As illustrated, the strength decreases linearly as the rubber content is augmented so that the strength drops from 263 kPa for the CS to 193, 154, and 85 kPa as the rubber increases to 10, 20, and 30%, respectively. The highest strength of 253 kPa was thus observed for the mixture containing 2% rubber powder. The observed reduction in the strength of the CS with increasing the rubber content can be related to the lower compaction of the clay–rubber mixture than the pure soil, leading to a fall in the strength of the mixture.

Figure 14c shows the modulus of elasticity of the clay–rubber mixture. The modulus of elasticity decreases whenever the rubber content is enhanced. As the rubber content increases to 10, 20, and 30%, the modulus of elasticity of the CS similarly declines from 96 kPa to 50, 35, and 27 kPa, respectively. In this respect, the highest modulus of elasticity of 85 kPa was observed for the mixture containing 2% rubber powder. The argument presented for the strength reduction also holds for the lower modulus of elasticity of the mixture.

Figure 14d displays the axial strain diagram based on the percentage of RWPs. It can be seen that increasing the amount of rubber increases axial strains. This trend is opposite to the trend of resistance changes with increasing rubber percentage. The failure strain in pure soil is equal to 4.1%. The amount of failure strains increases to 6.2, 8.1 and 10%, respectively, by increasing the amount of rubber by 10, 20, and 30%.

4. Conclusions

This study investigated the impact of rubber powder on the one-dimensional consolidation parameters of clay–rubber mixtures. Various geotechnical tests, including compaction and unconfined compressive strength (UCS) tests, were conducted to determine the properties of the mixtures. The main findings of this study are summarized as follows:

• Modified e-log p’ Curves: Modified e-log p’ curves were presented for the clay–rubber mixtures, considering the elasticity of the rubber powder and its influence on the void ratio (e) of the mixture. An equation was proposed to calculate the void ratio for the clay–rubber mixture, and the initial and modified e-log p’ curves were plotted;
• Swelling Potential and free swelling: The swelling potential of specimens containing rubber powder was lower than that of the natural clay, indicating a reduction in the swelling strain of the mixture. Free swelling also decreased with an increase in rubber content. For rubber contents above 20%, free swelling remained almost constant;
• Consolidation Parameters: The consolidation parameters Cc, Cs, and Cr increased with the rise in rubber content. The lowest values of Cs, Cc, and Cr were observed for specimens with lower rubber content, equal to 0.043, 0.01, and 0.225, respectively;
• Optimum Moisture Content and Maximum Dry Density: The optimum moisture content (OMC) increased while the maximum dry density (MDD) decreased with increasing rubber content. For rubber contents of 2% and 30%, the MDD and OMC were 16.3 kN/m³, 19.1%, and 13.2 kN/m³, 21%, respectively;
• Uniaxial Test Results: The uniaxial test revealed that lower rubber content provided the highest strength and modulus of elasticity, as well as the lowest soil settlement. Failure strains increased with higher rubber content, indicating the need for higher rubber content when greater flexibility in the mixture is required;
• Overall Impact of Rubber Content: Increasing the rubber content caused changes in soil parameters, such as increased failure strains and OMC and decreased free swelling, compression index, recompression index, swelling index, UCS, modulus of elasticity, and MDD.